1. Field of the Invention
The invention relates to a method for converting colours from one colour space, known as source colour space, into a colour space referred to as destination colour space.
2. Description of the Prior Art
Such a method may notably be used for the display of images by means of a device such as a plasma display screen or LCD (liquid crystal display) screen. Starting from source signals that express colour vectors in a source colorimetric space, independent of any device, such as for example a standard CIE space XYZ, or xyY, it is required that these colour vectors be expressed in a colour space particular to the display device, or destination space, and that the destination signals, capable of driving this device for displaying these colours, be thus obtained. For conventional display devices using three primary colours (red, green, blue), such a colour space particular to the device is generally denoted RGB and the components of each colour correspond to the control signal for each primary colour. This method is used in an analogous manner for printing an image, notably by means of a printer or film, and the three primary colours then generally correspond to cyan, magenta and yellow, the destination space then being denoted CMY; the space CMYK, where K denotes black, is preferably used.
Such a method may be used in reverse for capturing images by means of a device, such as for example a camera or a scanner. Starting from source signals delivered by the camera that express colour vectors in a colour space particular to the imaging device, or source space, generally denoted RGB, it is required that these colour vectors be expressed in a destination calorimetric space independent of any device, such as for example a standard CIE space XYZ, or xyY, and that signals capable, for example, of the transmission, storage and/or processing of the captured images be thus obtained.
In order to carry out such a conversion from one colour space-to another, a look-up table between certain colour vectors of the source space and certain colour vectors of the destination space is generally used; this table is generally obtained by a series of calorimetric measurements. This table may be seen as modelling the behaviour of the display or image capture device. To each row of this table, there corresponds a colour vector in the source colour space and in the destination colour space, and an end point of this vector which will be used as a node for the mesh of one and/or of the other space; each row of this table therefore contains the values of the components of the colour vector expressed in the source space and the values of the components of the same colour vector expressed in the destination space. By connecting the ‘nodes’ corresponding to the various-rows of this table, a mesh of one and/or of the other space is obtained, in such a manner that a mesh may be associated with any given colour vector of one or other space, to which mesh the end of this vector belongs.
In order to perform such a conversion from one colour space to another, interpolation means are generally then used whose general .principle is as follows: given a source colour vector to be converted, identified by the values of its components expressed in the source space, the mesh of the source space to which it belongs, the nodes that bound this mesh and the components of these nodes in the source space are identified, which are found in the rows of the look-up table; to this mesh of the source space there corresponds a mesh of the destination space, nodes that bound this mesh and components of these nodes in the destination space that are found in the same rows of the look-up table; starting from the position of the end of the source colour vector in the mesh of the source space, the position of the end of this vector in the corresponding mesh of the destination space can be deduced by interpolation, and since, thanks to the look-up table, the components of the nodes of this mesh expressed in the destination space are known, the components of the colour vector in this destination space are finally deduced; thus the values of the components of the destination colour vector sought are now able to be interpolated. It should be noted that the determination of the position of the end of the source vector in the mesh amounts to comparing the values of the components of the source colour vector to be converted with the values of the components of the colour vectors Which correspond to the nodes, expressed in the source space.
The documents U.S. Pat. Nos. 5,786,908, 6,034,667, 6,781,596, EP1223765 and US2005/128497 describe such methods based on look-up tables (LUT) and various interpolation algorithms, for example: tri-linear interpolation where each mesh comprises eight nodes, tetrahedral interpolation where each mesh comprises four nodes, prismatic interpolation where each mesh comprises six nodes.
Such conversions from one colour space to another require time and processing means that are sometimes too weighty for practical use. Indeed, colour spaces are generally at least three-dimensional, which makes the interpolation operations particularly complex.
In the case where the source and destination colour spaces each have a component correlated to the luminance, the other components then being chromatic components, the documents U.S. Pat. Nos. 5,650,942, 6,335,800 and EP1011263 teach that the .conventional three-dimensional interpolation operation is decomposed into a two-dimensional interpolation operation on the chromatic components and a scalar interpolation operation (one-dimensional space) on the component correlated to the luminance; each interpolation operation uses its own look-up table.
Such a decomposition advantageously allows the conversion calculations to be simplified and accelerated, notably because simpler look-up tables can be used. In practice, starting from any given source colour space, it is possible to perform, by a simple linear operation, a preliminary conversion towards another source colour space one component of which is correlated to the luminance, the other components then being chromatic components; thus, it is very easy to go from the space XYZ to the space xyY, where x=X/(X+Y+Z) and y=Y/(X+Y+Z), Y representing the luminance and x and y the tri-chromatic components; the reverse is true for the destination space. Consequently, the two-step interpolation method can be used for conversions between any given colour spaces, with the caveat of adding a preliminary linear conversion step between source colour spaces and/or a final linear conversion step between destination colour spaces.
One goal of the invention is to further improve the space conversion process described in the documents U.S. Pat. Nos. 5,650,942, 6,335,800 and EP1011263.